rule/trigger for batch update

Hi,
I want to make a trigger or a rule. However here is the tricky part: I want
it to happen after a batch of updates and not during one of them. It is
already set (by means of a unique index) that maximum one record can contain
the value 1. Point is that after every update I still have exactly one
record where the field value is 1. Here is what is supposed to happen in the
same batch:
UserRoleID (where value is 1) becomes 2 or 3. Which leaves no record to have
a 1 value in this field.
The field becomes 1 in another record.
Now both actions must happen or neither.

Here is the table creation code:
CREATE TABLE "tblUser" (
"UserID" serial NOT NULL,
"UserName" character varying(50) NOT NULL,
"UserPass" character varying(50) NOT NULL,
"UserRoleID" integer DEFAULT 3 NOT NULL
);
CREATE UNIQUE INDEX indsuperadmin ON "tblUser" USING btree ("UserRoleID")
WHERE ("UserRoleID" = 1);

Thanks in advance,
Nico.